1026
four successive increases in magnification reveals that as the
pixel spacing becomes smaller, the beam sampling footprint
eventually leaks into the surrounding pixels, so that the
beam no longer samples exclusively the region of a single
pixel. Eventually, when enough pixels overlap, the observer
will perceive this leakage as image defocusing or blurring.
The reality and limitations of this situation become obvious
when the microscopist seeks to perform high spatial resolu-
tion microscopy, a topic which will be covered in more depth
in module 10 on high resolution SEM.The effects of blurring are also encountered in the trivial
case when the objective lens is strengthened or weakened,
which moves the minimum beam convergence along the ver-
tical axis (either up or down), as shown schematically in. Fig. 6.11a, increasing the size of the beam that encounters
the specimen surface. The beam diameter that encounters the
specimen surface will be larger in either case because of the
finite convergence angle, α. As the beam samples progres-
sively more adjacent pixels just due to the increase in beam
size, and not dependent on the BSE-SE sampling footprint,
the observer will eventually perceive the defocusing, and
hopefully correct the situation!
Defocusing is also encountered when the specimen
has features that extend along the optic axis. For example,
defocusing may be encountered when planar specimens are
tilted or rough topographic specimens are examined, even
at low magnifications, i.e., large scanned areas, as illustrated
schematically in. Fig. 6.11b, c. In these situations, the diam-
eter of the converged beam that encounters the specimen
depends on the distance of the feature from the bottom of
the objective lens and the convergence angle of the beam, α.
Because the beam is focused to a minimum diameter at a spe-
cific distance from the objective lens, the working distance W,
any feature of the specimen that the scanned beam encoun-
ters at any other distance along the optic axis will inevitably
involve a larger beam diameter, which can easily exceed the
sampling footprint of the BSE and SE. . Figure 6.11d shows
an image of Mt. St. Helens volcanic ash particles where the
top of the large particle is in good focus, but the focus along
the sides of the particles deteriorates into obvious blurring,
as also occurs for the small particles dispersed around the
large particle on the conductive tape support. This defocus
situation can only be improved by reducing the convergence
angle, α, as described in Depth-of-Field Mode operation.
6.5 Making Measurements on Surfaces
With Arbitrary Topography:
Stereomicroscopy
By operating in Depth-of-Field Mode, which optimizes the
choice of the beam convergence angle, α, a useful range of
focus along the optic axis can be established that is sufficient
to render effective images of complex three-dimensional
objects.. Figure 6.12 shows an example of a specimen
(metal fracture surface) with complex surface topography.
The red arrows mark members of a class of flat objects. If the
microscopist’s task is to measure the size of these objects,
the simple linear measurement that is possible in a single
SEM image is subject to large errors because the local tilt of
each feature is different and unknown, which corresponds
to the situation illustrated in. Fig. 6.7. Although lost in a
single two- dimensional image, the third dimension of an
irregular surface can be recovered by the technique of ste-
reomicroscopy.123456789a1 2345 6789b. Fig. 6.10 a The beam sampling footprint relative to the pixel spac-
ing for a low magnification image with a low energy finely focused
beam and a high atomic number target. b As the magnification is
increased with fixed beam energy and target material, the beam sam-
pling footprint (diameter and BSE-SE convolved) eventually fills the
pixel and progressively leaks into adjacent pixels
Chapter 6 · Image Formation